1. Field of Invention
The present invention relates to a liquid crystal display and a dummy-loading device thereof. More particularly, the present invention relates to a dummy-loading device capable of providing edge band compensation and resolving unbalanced imaging of the liquid crystal display.
2. Description of the Related Art
Generally speaking, a liquid crystal display (LCD) can be roughly categorized into a passive matrix LCD and an active matrix LCD according to the driving method. A typical application for passive matrix LCDs are portable telephones. However, due to a relatively severe capacitor coupling effect, images produced by the passive matrix LCD device suffer from low quality, such as receding images, low contrast, slow response, etc. Moreover, passive matrix LCDs are generally driven by multiplexers, making their implementation more complicated as compared to active matrix LCDs. Accordingly, it is difficult for a passive matrix LCD to produce images with high resolution, high image quality, and full color. However, passive matrix LCDs are relatively inexpensive to produce. As such, they are used in low-end display apparatus.
On the other hand, laptop computers (notebook computers) or the monitors used in precision-made instruments, usually apply a thin-film-transistor LCD (TFT-LCD), an active matrix liquid crystal display. The active matrix LCD is an improvement over the passive matrix LCD, with better image quality and higher resolution, made possible by a drive array utilized to control the spinning of liquid crystal molecules.
FIG. 1 shows a schematic diagram of a drive array in a TFT-LCD. The drive array comprises a plurality of source lines 112˜118 (or data lines) for driving video data, a plurality of gate lines 132˜138 (or scan lines), a plurality of TFTs 152˜168, a plurality of liquid crystal capacitors 181˜197, and a coupling capacitance (not shown). A brief description of how much electric potential is applied to a liquid crystal molecule of each pixel in an LCD is given herein. In an active matrix LCD each pixel is controlled by one to four TFTs. In FIG. 1, a pixel is controlled by one TFT. The gates of the TFTs 152˜168 are connected in horizontal gate lines 132˜138, and the sources of the TFTs 152˜168 are connected in vertical source lines 112˜118, and the drains of the TFTs 152˜168 are connected to pixel electrodes. It should be noted that, in practical applications, the sources and the drains can be, but are not limited to being, connected to the data lines and the pixel electrodes, respectively. Also, the electric potential applied to electrodes of TFTs 152˜168 is not necessarily set at a constant value.
The operating method of a TFT is described in the following. First, a gate line is activated, such as gate line 132, to turn on all the TFTs 152˜156 of gate line 132. The desired video data to be displayed are inputted via the source lines 112˜118. The electrodes are charged to the electric potential corresponding to the video data. Next, the TFTs 152˜156 are turned off until the next video data is inputted, while the electric charges are preserved in the liquid crystal capacitors 181˜185. Thereafter, a next gate line is turned on, such as gate line 134, and desired video data are inputted. After the video data of a full image are inputted sequentially, the next image will be displayed starting from the first gate line. Because this driving method is quite simple, the interaction among pixels is substantially reduced. Also, the image quality of the LCDs can depend on the electrical characteristics of the TFTs. In this way, the cut-off current, the driving current, the parasitic capacitance, and the switching rate for the TFTs can determine the image quality of an LCD.
FIG. 2 schematically illustrates a conventional source driving circuit. The source driving circuit comprises a plurality of shift registers SR1˜SRn and a plurality of horizontal switches HSW1˜HSWn for driving a plurality of pixel bands B1˜Bn in an active area. As shown in FIG. 2, the sequential operation diagram of the video control signals is outputted from the shift registers SR1˜SRn. When the horizontal switches HSW1 and HSW2 are turned on, the video data are transferred to the pixel band B1. Accordingly, when video data of the 1st˜(n−1)th pixel bands B1˜Bn-1 are written in the active area, two horizontal switches are turned on simultaneously, equal to the loading effect of two pixels. When the video data of the last pixel band Bn are written, merely one horizontal switch HSWn is turned on, equal to the loading effect of one pixel. Different loading results in different electric potential of pixels and coupling capacitance. Consequently, in the prior art, when the last video data are transmitted, different loading leads to unbalanced imaging.